Wireless communication systems are continually striving to increase the data bandwidth so that information can quickly be exchanged between devices coupled to the communication system. Some of the parameters that limit the data bandwidth available to devices include the spectral bandwidth allocated to the devices and the quality of the channel linking the devices.
The spectral bandwidth allocated to communications in a wireless communication system typically is predetermined and does not vary. However, the quality of the channel may vary considerably for wireless communication devices. In wireless communication systems where at least one of the devices of a communication link is a mobile device, the channel quality can vary based on the relative positions of the devices.
The placement of objects and occlusions, and variations in the terrain between the devices can contribute to multipath effects that substantially degrade the channel quality. The distance between the communicating devices introduces signal attenuation that must be taken into account.
Wireless communication systems compensate for the various constraints on data bandwidth using a variety of techniques. A wireless communication system may incorporate multiple encoding techniques, and may select an encoding technique based on a data rate supported by a channel. In such a system, the communicating devices may negotiate a data rate based on the capabilities of the channel. Such a communication system may be advantageous for multiple point to point links, but may be less than ideal in a distributed broadcast system where a single transmitter provides substantially the same data to multiple receivers.
Other wireless communication system may incorporate hierarchical modulation, where multiple data streams are simultaneously transmitted. The multiple data streams can include a base layer that is a robust communication link capable of successful reception in nearly all receiver operating conditions. The multiple data streams can also include an enhancement layer that is broadcast at a data rate that is lower, the same, or higher than the data rate of the base layer. The communications over the enhancement layer may require a higher signal quality at the receiver compared to the base layer. Therefore, the enhancement layer may be more sensitive to variations in the quality of the channel.
In a hierarchical modulation system, the receiver is typically ensured the ability to communicate at the base level, and can typically demodulate data on the base layer. In channel conditions sufficient to support the enhancement layer, the receiver is also able to demodulate additional data modulated on the enhancement layer.
Regardless of the manner in which the wireless communication system transmits its data, a mobile device operating within the system typically operates from battery power. A mobile terminal can only maintain communications with another device if it has sufficient battery power to receive and demodulate received signals, and modulate and broadcast transmit signals.